What is the differential diagnosis for posterior lateral knee pain?

Differential Diagnosis for Posterolateral Knee Pain

Posterolateral knee pain has a distinct differential diagnosis that includes iliotibial band syndrome, popliteus tendon pathology, lateral meniscal tears, posterolateral corner ligament injuries, Baker's cysts, hamstring tendinopathy, and lateral compartment osteoarthritis, with iliotibial band syndrome being the most common cause in active individuals. 1, 2

Anatomic Structures and Associated Pathologies

Most Common Causes

• Iliotibial band (ITB) syndrome is the most frequent source of lateral knee pain, caused by inflammation of the distal ITB from repetitive knee flexion-extension, presenting as diffuse lateral knee pain that worsens with activity 2
• Lateral meniscal pathology including tears and ganglions can produce posterolateral pain, particularly involving the posterior horn 3, 4
• Baker's (popliteal) cysts commonly cause posterior knee pain and often communicate with the knee joint 1, 4
• Hamstring tendinopathy affecting the biceps femoris insertion can manifest as posterolateral pain 1, 4

Ligamentous and Capsular Injuries

• Posterolateral corner (PLC) injuries involve the lateral collateral ligament, popliteus tendon, popliteofibular ligament, and posterolateral capsule, requiring high clinical suspicion as they are frequently missed 5
• PLC injuries rarely heal with non-operative treatment when complete, necessitating surgical intervention in most cases 5
• Stress radiographs objectively determine the extent of PLC lesions when suspected 5

Less Common but Important Causes

• Popliteus tendon ganglions can cause persistent posterolateral pain, particularly after minor trauma, and may require surgical excision when conservative treatment fails 3
• Popliteus tendinitis is an uncommon cause that should be considered when other diagnoses are excluded 3, 4
• Lateral compartment osteoarthritis produces lateral-sided pain with characteristic radiographic findings 1
• Fabella syndrome can cause posterolateral pain, particularly in the context of total knee arthroplasty 6

Post-Surgical Considerations

• In patients with total knee arthroplasty, isolated lateral pain may result from soft tissue impingement against extruded cement, overhanging tibial tray, remnant osteophytes rubbing the ITB, or popliteal tendon impingement 6
• ITB traction syndrome secondary to guided motion bi-cruciate stabilizing knee arthroplasty represents a new clinical entity 6

Critical Diagnostic Pitfalls

Referred Pain Sources

• Hip pathology must be evaluated when knee imaging is normal, as hip disease commonly refers pain to the knee 7, 1
• Lumbar spine pathology should be considered when knee radiographs are unremarkable and clinical evidence suggests spinal origin 7, 1
• Thorough clinical examination of the lumbar spine and hip should precede knee-focused imaging 1

Imaging Considerations

• Initial evaluation requires plain radiographs (minimum anteroposterior and lateral views) before advanced imaging 7
• Approximately 20% of patients with chronic knee pain inappropriately receive MRI without recent radiographs, which should be avoided 7, 1
• Subchondral insufficiency fractures may show normal initial radiographs, later demonstrating articular surface fragmentation and subchondral collapse 1
• Ultrasound evaluation with elicited probe tenderness increases diagnostic accuracy for soft tissue pathology, and ultrasound-guided local anesthetic injections can confirm the pain source 6

Diagnostic Algorithm

Initial Workup

• Obtain anteroposterior and lateral knee radiographs first to exclude fractures, osteoarthritis, osteophytes, and loose bodies 7, 1
• Palpate specific structures: ITB at Gerdy's tubercle, lateral joint line for meniscal pathology, popliteal fossa for cysts, and biceps femoris insertion 2, 4
• Assess for PLC instability with varus stress testing and external rotation dial test 5

Advanced Imaging Indications

• If radiographs are normal or show only effusion but pain persists, MRI without IV contrast is indicated 7
• MRI excels at identifying meniscal tears, ligamentous injuries, bone marrow edema, popliteus tendon pathology, and Baker's cysts 8
• Consider hip radiographs if knee examination and imaging are unrevealing 7, 1

Confirmatory Testing

• Ultrasound-guided diagnostic injections can localize the pain source when clinical examination is equivocal 6
• Stress radiographs help quantify PLC injury severity when ligamentous injury is suspected 5